Hi Robert,
I can't speak to the 400, but today I shot a roll of 120 RPX 100 and a roll of APX 100 side by side.
I ran them both in Rodinal 1:25 for 9 mins and they seem pretty similar, with the Rollei looking a touch contrastier. I'll be really interested to print these up and see how the grain structure compares, I'd love to find an APX replacement before I shoot my remaining 70 rolls.
BTW I don't claim to be a scientist with this stuff, just reporting my experience today

Clearing up some of the confusion around Agfa APX and Rollei Retro films
The company that is going to reintroduce APX 100/400 is ADOX Fotowerke GmbH in Germany.
Adox bought a lot of the machinery from the old AGFA Leverkusen plant and hired some of AGFA Leverkusen's former engineers. (German information on the ADOX site here:- ADOX PAN 100). From what I've read in several places those films won't be made with the original recipes since Adox apparently doesn't own those but will be designed to be as close as possible. It won't be marketed under the APX name but under the name ADOX PAN 100 and 400 PROFESSIONAL. Those will be produced in 35mm, rollfilm and several sheet film formats. Release of the 100 iso films is planned for the first quarter of 2010.

Agfaphoto, the owner and producer of APX films went bankrupt in 2005, one year after is had been split of from the main company Agfa Gevaert in a management buy out. Agfaphoto is a brand name that has been licensed from Agfa Gevaert and since it's (Agfaphoto's) bankruptcy, does not produce anything, they just sell licences to photographic products manufacturers so they can sell their products under the Agfaphoto brand name.

Agfa Gevaert never went bankrupt and still produces film but NOT APX. They have no plans to reintroduce it and don't have the facilities to do so since the Leverkusen plant for APX production was sold to Agfaphoto and was sold off after the Agfaphoto bankruptcy.

Neither Lupus, nor O. Mahn GMBH (owner of the Maco and Rollei brands) nor Agfaphoto have any production facilities.

All the APX and Rollei Retro 100/400 still being sold is old stock. According to Adox several trading companies bought up old APX stock. I think they are refering to both O. Mahn GMBH (Rollei Retro 100/400) and Lupus (Agfaphoto APX 100/400). Adox also states that there is enough old stock around for it to be sold for several years to come. It seems that the Agfa APX film being reintroduced in the US is old stock owned by Lupus and sold under the licensed AGFAphoto brand name.

The Rollei replacements for Rollei Retro 100/400 (APX100/400) are Rollei Retro 80S and 400S which are not the same as APX films and are still being produced by Agfa Gevaert in Mortsel, Belgium and sold by Agfa (not to be confused with Agfaphoto) as aerial photography films (Aviphot, link here: AGFA.com - Aviphot.)

So good news for APX lovers! I'm curious to see the results from the first films and it will be great to have rollfilms on a less curly base than the Rollei Retro 100 rollfilm. That stuff is great but it curls like mad. I had to get a special anti newton glass sheet to scan them because they won't stay in my film holders.